In the late 19th Century the French physicist D'Arsonval suggested a closed cycle process, in which a working fluid such as ammonia is evaporated in a boiler heated by the warm surface sea water, used to drive an engine or turbine, and condensed in a heat exchanger cooled by the cold deep sea water supply. This process and its many related ones require that all the heat pass twice through heat exchangers: once to enter the working fluid and again to leave it. To keep the cost of the heat exchangers at a reasonable level, as much as half of the available temperature difference was consumed across the exchanger surfaces, leaving only half for power production in the working fluid. The limiting efficiency of the conversion to useful output is the Carnot efficiency, given by the ratio of the temperature range of the working fluid to the absolute temperature of its warmest point, or e=(T-T.sub.o)/T. Assuming a temperature of 25.degree. C. (77.degree. F.) or 298.degree. K. at the surface of the ocean, and 5.degree. C. (41.degree. F.) or 278.degree. K. at the ocean depths, the theoretical efficiency would be approximately 6.7%. A real heat engine works considerably below the Carnot efficiency, and some of the output energy or work must be consumed in pumping sea water, etc., so that an overall efficiency in the range of about 3% would be the expected result.
Another Frenchman, Georges Claude, avoided the large heat exchangers and their thermal losses by using the vapor of the sea water itself as the working fluid (See U.S. Pat. No. 2,006,985). The warm surface water was sprayed into an evacuated chamber, steam evaporated from the drops passed through a turbine to do work, and was condensed by direct contact with a spray of the cold sea water. The very low density of the steam required a large, lightly built turbine which was both expensive and fragile. Claude built an operating plant of this type on the coast of Cuba which produced 22 kilowatts of electrical power for a short time. Difficulties and cost of the turbine have generally discouraged interest in this type of plant.
Inventors have searched for a way to preserve the advantages and avoid the difficulties of the open cycle system. E. J. Beck (See U.S. Pat. No. 3,967,449) proposed that the vapor from warm water be used to lift the water itself against gravity. Bubbles of steam would be nucleated "in the water", and would produce a two-phase mixture of lower density than water, and could thus be lifted as in the well-known air-lift or gas-lift pumps used in the chemical and oil industries. Zener and Fetkovich proposed that the two-phase mixture of Beck be stabilized with a foam structure. In either case, the bubbles and the water containing the bubbles would "spill over" (in the terms of the Beck patent) into an elevated storage section. The water, once lifted, could be harnessed by means of a hydraulic turbine, instead of a steam turbine. The characteristic of these processes is that the two-phase mixture has water as the continuous phase and vapor as the dispersed phase, at least when the lifting action begins, and that the primary lifting effect is produced by balancing hydraulically a long column of lower density against a shorter one of higher density. Successive articles by Earl J. Beck "Ocean Thermal Gradient Hydraulic Power Plant", and by Clarence Zener and John Fetkovich "Foam Solar Sea Power Plant" appear at pages 293 through 295 of Vol. 189 of Science, in the July 25, 1975 issue.
There are major difficulties with these lift-pump processes. For the lift to succeed, the vapor pressure of the warm water at the bottom of the lift column must exceed the pressure at the top by at least the hydrostatic weight of the contents of the column, or the formation of vapor will be suppressed by submergence. Since the available vapor pressure range is low, the dispersed phase must greatly exceed in volume the continuous phase, or else the lift column must be relatively short, or both. It is very difficult to make such a light froth or foam remain stable over a large span or diameter. Beck's patent does not specify how the mixture is to be stabilized, but it is clear to anyone skilled in lift-pump practice that as the steam bubbles expand near the top of their travel (the pressure decreasing as they ascend), the continuous water phase may become discontinuous, so that slugs of water may fall back, while large slugs of vapor escape upward without lifting water. Usually contact with a nearby surface is required for stability. This would mean that such a lift pump would have to be filled with many parallel flow guides, such as a honeycomb structure, to insure stability of the mixture. This would tend to be as costly as a heat exchanger. The use of such a honeycomb flow path or of a foam-producing detergent as suggested by Zener and Fetkovich or both seem the only way to attack these problems, and these solutions create other problems in turn.
A different type of difficulty with these systems is that lifting the water upward from the sea surface, whence it falls again through a hydraulic turbine, necessitates a voluminous vacuum structure which must float high out of the sea, with accompanying problems of stability and windage. In addition, it is quite inconvenient to extract power from the two-phase vapor and water mixture which is obtained following the bubble lift pump step of Beck.
One object of the present invention is to obtain the advantages of the open cycle process for ocean thermal power while avoiding the problems of large steam turbines operating at supersonic vapor speeds on the one hand and the problems of unstable two-phase flow on the other. A further objective of the invention is to provide a technique whose natural configuration lends itself to a compact, economical, sea-worthy, and low-profile arrangement well suited to practical sea-going use.